This invention relates to apparatus for monitoring a plurality of input lines to detect the presence of analog signals. More particularly, it is concerned with scanners for detecting supervisory tones on telephone lines.
In a telephone exchange it is necessary to continuously monitor incoming lines to detect requests for service. Certain telephone systems employ supervisory tones of various predetermined frequencies for signaling for service. In a typical system an incoming line desiring service may have present thereon one or two tones of audio frequency indicating the nature of the service being requested. The function of continuously monitoring the incoming lines to detect the presence of these supervisory tones is performed by the scanner which recognizes the particular tones and the particular lines on which the tones are present.
It is common practice for the incoming lines to a scanner to be arranged in groups, for example, groups of 12 lines. The 12 lines of a group are connected to a multiplexer and the lines are sampled continuously, for example, for about 17 milliseconds of every 200 milliseconds. In a typical arrangement the output of the multiplexer is passed through a high pass filter and applied to a pair of band reject filters to separate high frequency tones and low frequency tones. The outputs of the band reject filters are applied to comparator and limiter circuitry. The resulting signals are applied to a bank of tone filters each of which passes one of the predetermined frequencies used in the system. The output of each tone filter is connected to a level detector which is set by a signal passing through its tone filter and produces a signal indicating that a particular tone is present on the line being sampled.
The scanner as briefly described requires complete duplication of all the scanning equipment for every group of 12 lines. The total amount of equipment employed increases proportionally with the number of groups of incoming lines in the system. Similarly, the space requirements, weight, and cost of a system also increases proportionally with the number of lines.